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Glossary 507 Coupling The splitting of an NMR spectral peak by neighbors into more complex patterns. Covalent bond A bond formed by the sharing of electrons between atoms-in contrast to an ionic bond. Cracking The breaking of alkanes into smaller fragments, as in refining crude oil. Cross-linkage The connection of two or more polymer chains; gives rubber its familiar hardness and elasticity. Crossed aldol condensation An aldol condensation of two different aldehydes or ketones. Cyano group -CN, the functional group of the nitriles. Cyanohydrin NC-RR'COH, the product of hydrogen cyanide addition to the carbonyl group. Cyclic bromonium ion An intermediate in addition reactions, in which bromine bridges both carbons of a double bond. Cycloaddition An addition reaction, such as the Diels-Alder reaction, leading to a closed ring (the "cycloadduct"). Cycloalkane An alkane whose carbons form a ring, also called a cyclic alkane or carbocycle. Cycloalkanecarboxylic acid A saturated cyclic carboxylic acid. Cycloalkanone The systematic name for a cyclic ketone. Cytosine (C) One of the four bases in both DNA and RNA, a polyfunctional nitrogen heterocycle. D, L sugars R, S configurations based on the highest-numbered stereocenter, with all natural sugars D. Dashed-wedge line notation Molecular representation using dashed lines for bonds angled below the page, wedges for bonds angled above it. DCC a dehydrating agent in peptide synthesis. Deactivating group Substituents allowing only sluggish reaction, such as electron-withdrawing groups on benzene undergoing electrophilic attack. Decoupling Techniques that remove NMR coupling, such as proton decoupling and fast proton exchange. Degenerate orbital Orbitals of equal energy, such as the three orbitals, and Degree of unsaturation The sum of the number of rings and bonds in a molecule, determined from the molecular formula. Dehydration Elimination of a molecule of water. Delocalization The distribution of electrons over several nuclei, as in a resonance hybrid. Deoxyribonucleic acid (DNA) The dimeric helix of nucleotide chains that carries hereditary informa- tion-the genetic code. Deshielding An increase in the local magnetic field or removal of electrons from a nucleus, causing a down- field NMR chemical shift. Dextrorotatory molecule An enantiomer that rotates the plane of polarization clockwise as the viewer faces the light. Diamine A compound with two amine groups, such as 1,4-butanediamine (putrescine). Diastereomers Stereoisomers that are not mirror images of one another. 1,3-Diaxial interaction Transannular strain due to steric crowding of axial substituents across a ring. Diazo compound A diazoalkane, R₂C=N₂, such as diazomethane a useful synthetic intermediate. Diazo coupling Electrophilic substitution with arenediazonium salts, giving azo dyes. Diazonium ion a highly reactive product of the reaction of a primary amine with nitrous acid. Diazotization Synthesis of arenediazonium salts from benzenamines (anilines) using cold nitrous acid. B-Dicarbonyl compound A useful intermediate bearing two carbonyl groups adjacent to the same carbon. Dieckmann condensation Intramolecular Claisen producing cyclic 3-keto esters. Diels-Alder reaction Concerted + 2] cycloaddition of dienes, with four TT electrons, and alkenes, with two, to form cyclohexenes. Diene A compound containing two double bonds. including conjugated and nonconjugated dienes as well as allenes.